The present invention generally relates to the formation of a relaxed or pseudo-relaxed layer on a substrate. The relaxed layer may be made of a material selected from semiconductor materials, in order to form a final structure for electronics, optics or optoelectronics, such as a semiconductor-on-insulator structure.
A layer is “relaxed” if its crystalline material has a lattice parameter substantially identical to its nominal lattice parameter, wherein the lattice parameter of the material is in its bulk equilibrium form. Conversely, a layer is “strained” if its crystalline material is elastically stressed in tension or in compression during crystal growth, such as during epitaxy, which forces its lattice parameter to be substantially different from its nominal lattice parameter.
Methods for forming a relaxed layer on a substrate are known. A method for doing so includes conducting epitaxial growth of a thin layer of semiconductor material on a donor substrate, bonding a receiver substrate at the thin layer; and then removing a part of the donor substrate. A semiconductor-on-insulator structure can thus be made. The semiconductive thickness consists partially of the relaxed thin layer, and the insulating layer is usually formed in an intermediate step between the epitaxial and bonding steps.
The thin layer may be fabricated during an epitaxial step or during subsequent treatment. In the first case, it is known to use a donor substrate consisting of a backing substrate and a buffer layer, the buffer layer confining plastic deformations so that the epitaxial thin layer is relaxed from any stress. Such methods are, for example, described in published documents US 2002/0072130 and WO 99/53539. However, a buffer layer is often time consuming and costly to make. In the second case, the donor substrate does not comprise any buffer layer and the epitaxial step then consists of growing the thin layer to be stressed by the donor substrate. Thus, for example, a SiGe layer will be grown directly on a Si substrate that has a thickness such that the SiGe layer is globally stressed.
A first technique for relaxing the SiGe layer, notably as described in the document of B. Hollander et al. entitled “Strain relaxation of pseudomorphic Si1-xGex/Si(100) heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication” (in Nuclear and Instruments and Methods in Physics Research B 175-177 (2001) 357-367) consists of relaxing the SiGe layer, before applying a bonding step, by implanting hydrogen or helium ions in the Si substrate at a predetermined depth. However, relaxation rates usually obtained with this first technique remain rather low as compared with other techniques.
A second technique is disclosed in the document entitled “Compliant Substrates: A comparative study of the relaxation mechanisms of strained films bonded to high and low viscosity” by Hobart et al. (Journal of Electronic Materials, vol. 29, No. 7, 2000). After removing the donor substrate during a removal step, heat treatment is applied for relaxing or pseudo-relaxing a layer of stressed SiGe, bonded to a BPSG glass during the bonding step. During the heat treatment, the stressed layer thus seems to relax via the layer of glass which has become viscous due to the treatment's temperature. However, this latter technique involves relaxation of the SiGe thin layer when the latter is exposed. Exposure of such a SiGe layer (exposed) to a gas atmosphere during heat treatment (such as a room RTA treatment, a sacrificial oxidization, or a recovery anneal) may prove to be disastrous for the quality of this layer, wherein Ge contained in the layer may diffuse outwards (which may cause decomposition of the layer) and the layer may be contaminated by external contaminants.
Furthermore, such a SiGe layer is on the surface and may therefore have to undergo special treatment such as finishing treatments (polishing, smoothening, oxidization, cleanings, etc.). At the present, such treatments for SiGe are not as effective as those for Si. This lack of control when working with SiGe causes further difficulties for making a desired structure.
The present invention now overcomes these problems.